This invention relates to a process for the melt-spinning of a filamentary structure from a synthetic polyamide polymer. More particularly, it is concerned with a continuous process for the formation of high strength polycaproamide filament, yarn or the like by melt-spinning a synthetic linear fiber-forming polycaproamide having excess number of carboxyl and groups over amino end groups.
U.S. Pat. No. 2,174,527 discloses that dibasic dicarboxylic acids when employed in excess in a diacid-diamine polyamide, serve to terminate the polymer, thereby minimizing further increase in polymer molecular weight.
U.S. Pat. No. 2,241,321 teaches polymerization of caprolactam in the presence of a diamine, followed by addition of sebacic acid and completion of the polymerization. The sebacic acid apparently acts as a chain-extending agent.
More recently, U.S. Pat. No. 3,386,967 discloses that a high strength polycaproamide yarn having an excess number of carboxyl end groups over amino end groups may be produced from a high molecular weight polymer prepared by polymerizing e-caprolactam and reacting the polymer with a dibasic carboxylic acid containing at least six carbon atoms. Because polycondensation of the polymer in the presence of a dibasic carboxylic acid is relatively slow, the acid is desirably included at the start of the polymerization reaction. Even so, equilibrium is approached in the polymerization reaction mixture at number average molecular weight not above about 20,000 under usual polymerization conditions, and special measures are required to carry the reaction further. One particularly useful method of accomplishing the required further reaction is to remove volatile by-products of the polymerization such as water by flowing an inert gas across the reaction mixture surface, desirably followed by application of vacuum.
Although the process of U.S. Pat. No. 3,386,967 constitutes a major contribution to this art, we have found that in continuous operation of the process on a commercial basis, particularly at maximum production rates, serious problems have been encountered in melt spinning due to the frequent occurrence of "nubs" in the fiber. The term "nubs" is conventionally applied and is used herein to mean enlarged sections of filament no more than several filament diameters in length. Nubs may be formed by a foreign, nonorientable substance which interferes with normal fiber stretch in a short section, resulting in an enlargement. Foreign substances which are believed to have contributed to nubs in the present instance include carbonized polymer and gels formed in the polymer. Gels appear to be the chief cause, i.e., the nubs are probably created by non-orientable gel from cross-linked polymer. Thermal degradation of the polymer may be an important causative factor.
The reactions in thermal degradation of polyamides containing dicarboxylic acid additives are not entirely understood. It is likely that thermal degradation produces a decomposition product which serves to form cross-links between amide groups and adjacent polymer chains. The decomposition reaction proceeds slowly, finally building up a three-dimensional network of molecules which may be called polymer gel and which eventually reaches the stage where it forms an infusible coating on the walls of the equipment.
A serious difficulty which arises from the formation of this polymer gel on the interior walls is that from time to time pieces break off and get into the flowing polymer stream where they produce damage to the spinning equipment.
The greatest difficulty, however, is caused by polymer gel which has progressed to the three-dimensional structural stage, but which has not yet reached the stage of being infusible. This kind of polymer gel is readily carried with the stream of flowing polymer. Being still molten or at least softened, it passes through the pump and even through the filter medium to show up either as discontinuities or as viscosity differences in the spun filament. When these filaments are later drawn, these defects may cause breaks in the filaments which either cause the whole thread to break or else form nubs which go through to be counted as quality defects in the final yarns.